There is a need in the industry for scaleable processes that are capable of generating electrophilic fluorination (F+) agents with sufficient F+ character, or alternatively, F+ power, to affect electrophilic fluorination reactions of a wide variety of organic substrates with high selectivity, and are also safe and economical to produce and use.
Examples of F+ agents which possess sufficient F+ power and selectivity in fluorination reactions with organic substrates include bis(fluoroxy)difluoromethane (BDM) and fluoroxytrifluoromethane (FTM.) BDM and FTM can be produced in a cesium fluoride (CsF)-catalyzed reaction between fluorine and carbon dioxide or fluorine and carbonyl fluoride, as shown in the reaction schemes below. 
The preparation and handling of the CsF catalyst is critical to its effectiveness as a catalyst. The CsF must be rigorously dried and totally free of water and hydrogen fluoride. Even a minimum exposure of the bulk catalyst bed to either water or hydrogen fluoride can result in an immediate and irreversible loss of catalytic activity.
One of the major problems in using a CsF catalyst is preparing CsF in an anhydrous condition, and subsequently loading a vessel with the dry material while preserving the anhydrous condition. A known method for preparing the CsF catalyst is to melt CsF at 800xc2x0 C. and then allow the molten material to cool and solidify in a moisture-free glovebox. The solidified material is then finely ground using a combination of mortar/pestle and electric grinder. The catalyst material so prepared is transferred, inside the dry atmosphere of a glovebox, to the reactor.
A problem with the above method of catalyst preparation is the fine powder form of the activated catalyst material. When loaded into the catalyst vessel, the fine powder can create an unacceptable pressure drop across the vessel. This becomes especially problematic at the high gas flow rates that are required in large scale BDM and FTM technologies. Another problem arises when performing the above described catalyst preparation procedure on the scale required for commercial scale use. Commercial scale catalyst vessels can be too large and unwieldy to be handled in a conventional drybox. A further problem with the above described catalysts is the potential for channeling which compromises efficiency of the catalyst bed.
Other fluorides, such as potassium fluoride and sodium fluoride, have been shown to catalyze specific fluorination reactions.
Examples of processes for producing BDM and FTM using fluoride catalysts are described in the following publications:
Frederick A. Hohorst and Jean""ne M. Shreeve (Journal of the American Chemical Society, V.89, (1967), pages 1809-1010,) describe the static fluorination of carbon dioxide with a large (305%) molar excess of fluorine in the presence of a large (18 mmoles) molar excess of cesium fluoride to prepare BDM.
Ronald L. Cauble and George H. Cady (Journal of the American Chemical Society, V. 89 (1967), page 1962) describe a preparation of BDM in which carbon dioxide was reacted with nearly 100% excess fluorine in the presence of pulverized CsF as the catalyst.
Max Lustig, et al. (Journal of the American Chemical Society, V.89 (1967) pages 2841-2843) describe a low temperature preparation of fluoroxy compounds, including BDM, by catalytic fluorination of carbonyl halides and fluoroalkyl acid fluorides. Finely ground cesium fluoride was used in the examples.
U.S. Pat. No. 3,394,163 (Kroon, 1968) discloses preparation of BDM by treating an alkali metal oxalate with fluorine in the presence of an alkali metal fluoride or an alkaline earth metal fluoride catalyst.
U.S. Pat. No. 4,499,024 (Fifolt, 1985) discloses a continuous reaction for preparing BDM by reacting carbon dioxide and fluorine in the presence of CsF catalyst. The reactants are passed through a reactor, such as a nickel or nickel-lined tube containing particles or powder of cesium fluoride. In the examples, a 4:1 mole ratio of fluorine:carbon dioxide is used.
Michael J. Fifolt, et al. (Journal of Organic Chemistry, V.50 (1985) pages 4576-4582) disclose the production of BDM and FTM by the reaction of fluorine with carbon dioxide and carbon monoxide, respectively, using a cesium fluoride catalyst. It is reported that preparation of cesium fluoride by fusion and subsequent grinding under anhydrous conditions is critical for the reaction to occur.
U.S. Pat. No. 2,689,254 (Cady and Kellogg, 1954) discloses a process for preparing FTM by reacting carbon monoxide and fluorine in the presence of a catalyst comprising a copper ribbon coated with silver fluoride.
R. Craig Kennedy and George H. Cady (Journal of Fluorine Chemistry, V.3 (1973/74), pages 41-54,) report on the use of several fluoride catalysts, such as sodium fluoride, ammonium fluoride, and barium fluoride, in the preparation of FTM. Except for the fluorides prepared in the reactor, each catalyst was ground to a fine powder before it was added to the reactor.
The need remains for an effective catalyst that can be used in large scale fluorination processes; for example, a catalyst that will not create a large pressure drop across a vessel used in large scale production of BDM or FTM.
This invention is directed to new active fluoride catalysts that are useful in producing electrophilic fluorination agents. The new active fluoride catalysts are especially useful in producing BDM from the reaction of fluorine with carbon dioxide and FTM from the reaction of fluorine with carbonyl fluoride. The fluoride catalyst comprises a mixture of two or more fluorides selected from a transition metal fluoride, an alkali metal fluoride and an alkaline earth metal fluoride. Alternately, the fluoride catalyst is one or more fluorides, such as alkali metal fluorides, alkaline earth metal fluorides, and/or transition metal fluorides, deposited on an inert support, such as a zirconium oxide (ZrO2) support. The catalysts of this invention are in the form of particles or pellets such that the surface area is preferably at least about 0.1 m2 per gram.
In an embodiment of this invention, the active fluoride catalysts are formed by depositing an aqueous mixture of one or more of an alkali metal salt, an alkaline metal salt, and/or a transition metal salt on an inert support, heating the supported salts to evaporate the water, thoroughly drying the supported salts under dry nitrogen, and activating the salts by passing fluorine or fluorine-containing gas over the supported material to convert the salt to a fluoride.
In another embodiment of this invention, the active fluoride catalysts are formed by mixing together, in an aqueous medium, a transition metal salt, preferably a fluoride, and one or more of an alkali metal salt, alkaline earth metal salt, and/or another transition metal salt, heating the mixture to evaporate the water, extruding the dried mixture, breaking the extrudate into particles or pellets, thoroughly drying the particles under dry nitrogen, and activating the salts by passing fluorine or fluorine-containing gas over the supported material to convert the salt to a fluoride.
In another embodiment of this invention the above catalysts are used in fluorination reactions in which BDM is formed from the reaction of fluorine and carbon dioxide or FTM is formed from the reaction of fluorine and carbonyl fluoride.
In another embodiment of this invention, the above catalysts are formed in situ prior to reacting fluorine and carbon dioxide to form BDM or fluorine and carbonyl fluoride to form FTM.
The fluorination catalysts prepared by the method of this invention provide the following advantages compared to known catalysts:
eliminates the need for large scale melting and grinding equipment in making the catalyst;
the size of the catalyst particles prevents high pressure gradients in large scale fluorination reactors;
the catalysts can be made and activated in situ prior to using them in a fluorination reaction;
use of the catalysts in situ reduces or eliminates the problems associated with handling hygroscopic fluorides under anhydrous conditions;
little or no fluorine or byproducts are produced in the fluorination reaction; and
less catalyst is needed for fluorination reactions.
The active fluoride catalysts of this invention can be prepared by depositing, on an inert support, an aqueous mixture of one or more salts of an alkali metal, an alkaline earth metal, and/or a transition metal. Examples of appropriate salts are carbonates, bicarbonates, oxides, halides, sulfates, phosphates, nitrates, or hydroxides. Examples of alkali metal, alkaline earth metal, and transition metals are cerium, cobalt, cesium, potassium, sodium, rubidium, lithium, beryllium, magnesium, calcium, barium, and strontium. Examples of inert supports are zirconia, alumina, titania, magnesia, clays, aluminosilicates, and silica.
The wet supported material is heated to a temperature of about 100xc2x0 C. to evaporate most of the water. It is then thoroughly dried by heating it up to 800xc2x0 C. under a flow of a dry inert gas, preferably dry nitrogen. In some cases, heating to a temperature that results in decomposition of the salt to an oxide, during drying, is preferred.
Alternately, the catalysts of this invention can be prepared by mixing a transition metal salt, preferably a fluoride such as cerium fluoride, with one or more salts of an alkali metal, an alkaline earth metal, and/or another transition metal. The salt can be any of the salts listed above for the preparation of a supported catalyst. The salt and fluoride are mixed together in an aqueous medium, preferably water, and dried in an oven at a temperature of 100 to 150xc2x0 C. The mixture can then be extruded using known extruding equipment, to form an extrudate that is typically about xe2x85x9 inch in diameter. The extrudate is thoroughly dried again under an inert gas, preferably nitrogen, at a temperature of at least 100xc2x0 C. and broken into smaller particles, for example, pelletized, such that the surface area of the particles is preferably at least 0.1 m2/g. The particles are then dried again under nitrogen at a temperature of at least 400xc2x0 C.
The catalysts of this invention can also be prepared from a mixture of one or more salts of alkali metals, alkaline earth metals, and/or transition metals by a method as described above.
The dried catalytic material are activated by passing fluorine or fluorine-containing gas over the catalytic material to completely convert the salts to fluorides. The fluorine gas can be undiluted fluorine, or fluorine diluted in an inert gas such as helium, neon, argon, krypton, xenon, nitrogen, carbon dioxide, sulfur hexafluoride, tetrafluoromethane, or nitrogen trifluoride. Alternatively, the fluorinating gas can be sulfur tetrafluoride, nitrogen trifluoride, xenon difluoride, krypton difluoride, oxygen difluoride, or dioxygen difluoride.
Activation of the supported catalysts can be carried out at temperatures ranging from xe2x88x9278 to 150xc2x0 C. Temperatures of xe2x88x9278 to about 300xc2x0 C. can be used in the activation of the unsupported catalysts. Pressure can range from sub-ambient (vacuum) to super-ambient (up to about 300 psig (2170 kPa)) during activation.
The catalysts of this invention are in a form that provides low back pressure in a fluorination reactor; for example, pellet, extrudate, sphere, tablet, and honeycomb. Pellets or granules having a surface area of at least 0.1 m2/g are preferred. The above fluorination catalysts are useful in the preparation of fluorination agents. They are especially useful in preparing BDM by the reaction of fluorine and carbon dioxide or FTM by the reaction of fluorine with carbonyl fluoride in a continuous process. The catalysts can also be produced in situ prior to introduction of the fluorine and carbon dioxide for the preparation of BDM or fluorine and carbonyl fluoride for preparing FTM.
In the preparation of BDM, a gas flow of fluorine and carbon dioxide can be passed through a catalyst bed containing a catalytic effective amount of catalyst. The molar ratio of carbon dioxide to fluorine can range from 0.5 to 25; preferably 2.5 to 10.
By catalytic effective amount of catalyst is meant an amount which will convert all available fluorine to product. The amount of catalyst that is required depends on the intended flow rate of reactant gases, the catalyst bed pressure used, the ratio of CO2, F2, and other process parameters.
The temperature in the reactor can range from xe2x88x9250 to 100xc2x0 C., preferably xe2x88x9210 to 50xc2x0 C., and the pressure can range from 0 to 400 psig (101 to 2859 kPa), preferably 1 to 300 psig (108 to 2170 kPa). Reaction conditions are adjusted so that the product gas contains little or no detectable fluorine or byproducts.
In the preparation of FTM, a gas flow of fluorine and carbonyl fluoride can be passed through a catalyst bed containing a catalytic effective amount of catalyst. The molar ration of carbonyl fluoride to fluorine can range from 1 to 5; preferably 1 to 2.
The temperature in the reactor can range from xe2x88x9250 to 100xc2x0 C. (preferably xe2x88x9210 to 50xc2x0 C.) and the pressure can range from 0 to 400 psig (preferably 1 to 300 psig). Reaction conditions are adjusted so that the product gas contains primarily FTM with little or no detectable fluorine, COF2, or byproducts.
If needed, BDM and FTM can be readily separated from other reaction products by known techniques, such as liquefaction.
The invention will be further clarified by consideration of the following examples, which are intended to be purely exemplary of the invention.